An expanded deep-branching thermophilic bacterial clade sheds light on the early evolution of bacteria

The origin of thermophilic bacteria is essential to our understanding of the early evolution of life. However, due to the lack of deep-branching culturable lineages, many controversies remain. Here, we report a novel deep-branching, sulfur-reducing, thermophilic bacterium that was isolated from a deep-sea hydrothermal vent using a newly developed cultivation strategy ("Subtraction-Suboptimal", StS). This bacterial lineage clusters together with other major thermophilic bacterial groups on phylogenomic trees and forms a robust superphylum-level clade that represents a major, early-diverging bacterial lineage. Ancestral analyses and metabolic modeling reveal the ancestor of this lineage might be a thermophilic and mixotrophic bacteria with a preference for amino acids utilization. These findings provide evolutionary insights into the early divergence of thermophilic bacteria and their adaptive strategies on early Earth.